demandimport: disable if chg is being used
In chg's case, making modules lazily loaded could actually slow down things
since chg pre-imports them. Therefore disable demandimport if chg is being
used.
This is not done by setting `HGDEMANDIMPORT` chg client-side because that
has side-effects on child processes (hooks, etc).
Differential Revision: https://phab.mercurial-scm.org/D351
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